1. Field of the Invention
The present invention relates to a developer for use in electrophotography and a method and an apparatus for image formation, in particular, relates to a carrier and a toner for use in such a image formation method as electrophotography, a developer which uses the toner and carrier, a method for image formation which method uses the toner and developer, and an apparatus for image formation which apparatus uses the toner and developer.
2. Description of the Related Art
A two-component developer for use in electrophotography is composed of a toner and a carrier, and the carrier is a conveying substance which is mixed with the toner under stirring in a development box to transfer the charge to the toner, and delivers the charged toner to an electrostatic latent image on a photoreceptor to form a toner image. The carrier is left on a magnet roll and then returns to the development box where the carrier is again mixed with the toner under stirring to follow repeatedly the same process as mentioned above. Accordingly, the carrier is required to acquire and maintain the desired charging characteristics in relation to the toner in any environment.
Conventional developers have two problems that are caused by the mutual collision of the carrier particles through stirring and by the friction between the development box wall and the carrier particles: One is the problem that the toner fuses and sticks to the carrier particle surface and hence the toner is made to be spent; and the other is the problem that the resistance of the carrier is degraded by the peeling off and elimination of the coated resin layer on the carrier particle, inducing the degraded images (image density deficiency, image fogging defect, and the like) after plate wear, as compared with the images in the earlier stages of the printing. The degradation of the developer (carrier) due to stirring as well as the environmental variation at the time of plate wear induces a reduction of the charge when the ambient temperature and humidity are high, resulting in toner scattering and image fogging defect. On the other hand, as matters now stand, when the ambient temperature and humidity are low, charge increases to cause a problem of image density deficiency, eventually resulting in a reduced life of the developer. In particular, as the carrier for use in full color printing, as compared to the printing with the conventional copying machines, printers, and the like, the solid printing areas are larger than the character printing areas so that the toner quantity transferred to the photoreceptor becomes large and hence the further higher durability is required to the carrier.
As measures to overcome such toner spent and the degradation of the charge due to the environmental variation at the time of plate wear, a variety of methods have been proposed for coating the carrier particle with resins. No satisfactory method, however, has been developed yet. For example, the carriers coated with the silicone based resins and the fluororesins have low critical surface tension, so that there scarcely tends to occur the spent state in which the toner fuses and sticks onto the surface of the carrier particle, resulting in elongation of the life of the developer. On the other hand, the carriers just mentioned above are low in the ability of charging the toner and carrier so that the toner scattering frequently occurs to contaminate the interior of the printing machine and to eventually induce the defects on the image, leading to the overall evaluation that the developer is still short in life.
The use of the silane coupling agent has been proposed (see Japanese Patent Laid-Open No. 60-19156), as the measures to overcome the image degradation (image density deficiency, image fogging defect, and the like) due to the peeling off and elimination of the coated layer on the surface of carrier particle caused by the mutual collision of the carrier particles through stirring and by the friction between the development box wall and the carrier particles. The use of the silane coupling agent improves the coated layer adhesion, but there is still the problem that the charge variation under the various environmental conditions causes the toner scattering and induces the image fogging.
On the other hand, there has been proposed a carrier and the like which carrier is provided with a coating layer composed of silicone resin on the surface of the carrier core processed with the silane coupling agent for the purpose of improving the adhesion between the carrier core and the silicone resin (see Japanese Patent Laid-Open No. 62-121463). This carrier, however, has no aminosilane coupling agent component having effective amino groups on the outermost surface of the carrier, and hence the carrier has no sufficient charging ability in relation to the toner negative in electrode property, resulting in the toner scattering to occur at the time of plate wear, namely still leading to unsatisfactory results.
Recently, there has been described a carrier in which the resin layer containing the aminosilane coupling agent comprises the outermost layer and intermediate layer different from each other in resin composition or in additive (see Japanese Patent Laid-Open No. 5-72815, and Japanese Patent Laid-Open No. 5-134467). Furthermore, there has been described a carrier in which a concentration gradient is provided along the thickness direction of the silicone resin layer with respect to the silane coupling agent and the like (see Japanese Patent Laid-Open No. 5-204189). In these carriers, the individual constituent components are not homogeneous in the resin layer of the carrier, and hence, in particular in the silicone resin coated carrier, there occurs the time variation in the carrier consititution during being allowed to stand in such a way that the outermost layer and the intermediate layer of the resin layer deviate from each other in curing state. Hence the early charging property as prepared in relation to the toner and the charging property after an elapse of time in relation to the toner differ largely, the carrier containing a conductive material that decreases in charge with high humidity, and furthermore, when the peeling off and elimination of the resin layers occur at the time of plate wear, the resistance of the carrier alters largely, so that these carriers cannot be said to be satisfactory in durability in a conclusive estimation.
In the conventional resin coated carrier as described above in which the aminosilane coupling agent is used, the content of the aminosilane coupling agent is in most cases 3 wt % or below in relation to the coated layer resin, and at most 5 wt % in relation to the resin as described in Japanese Patent Laid-Open No. 61-140951.
Nowadays, demand is increasing for the uniform reproduction of the images such as barcodes and the like having solid printing portions in large proportion and the images of the graphic designs and the like, in place of the vouchers carrying mostly characters printed by printers and the like; in particular, in full color printing and the like, the solid printing areas are larger than the character carrying areas, and accordingly the consuming amount and the feeding amount of the toner are increased. It is demanded that the toner always maintains a desired charging characteristic in the development process of electrophotography under any environmental conditions. At present, however, in light of the recent development conditions of the high amount toner consuming and feeding system, there have been obtained no satisfactory developer and carrier. In other words, in particular, the conventional toner is not yet satisfactory with respect to the rise behavior of the charging property under the conditions of high temperature and high humidity, although the conventional carrier and developer are improved in preventing the toner from being spent, in adhesion property of the coated layer of the carrier, and to some extent in the charge donating ability of the carrier to the toner through the use of the aminosilane coupling agent. At present, the carrier has no sufficient ability of charge donation to the small-sized particle toner and the high density toner for use in the high-resolution image printing, recently involving in particular large areas of solid printing, on the printers and full color machines. Hence, the carrier cannot quickly raise the charge in the toner which is fed at the time of plate wear, which means the durability of the developer is still unsatisfactory.